You said you want f.lux to "start" at 8pm. I interpret that as, "I want Night mode to begin at 8pm". So, that means you want Daytime mode to end at 8pm. However, now you're saying you want Daytime to begin at 8pm?

What exactly do you want? Just give me all the details (bury me in all of the details, even if it means your post takes 10 minutes to read, I don't mind), and I will try to tell you how to accomplish your goal. I don't let f.lux do anything automatically, so I'm sure I can help you.

Forget taking a screenshot. Again, a screenshot cannot capture this kind of thing (at least, it shouldn't be able to). It can only be captured with a photo that you take with your camera. Regardless, I really don't think I need to see any kind of screenshot to see what you're saying. This is good enough for me.

@twocables Hello ! Thanks for your answer ! Sorry for my very late answer, i had shift the topic thinking there was no solution.
My trouble is the light beguins to change too earlie : i would like the daytime end not before 8 PM. Sorry i cant make more clear (my english limited, not my mother tong)
Hope you can help me !